1. Field of the Invention
This invention relates to an apparatus for synthesizing laser beams, and more particularly to an apparatus for synthesizing laser beam bundles respectively radiated from a plurality of semiconductor lasers into a synthesized laser beam bundle and converging the synthesized laser beam bundle into optical fiber.
2. Description of the Related Art
There has been known a technology in which bundles of laser beams respectively radiated from a plurality of semiconductor lasers arranged in one direction are passed through a collimator lens into parallel beam bundles (having parallel optical axes) arranged in one direction, and the whole parallel beam bundles are condensed and introduced into a single optical fiber, whereby a laser beam of a high energy density is caused to propagate the optical fiber. See, for instance, U.S. Patent Laid-Open No. 20020090172.
In the technology, though the beam synthesizing efficiency (the efficiency of synthesizing the beam bundles radiated from a plurality of semiconductor lasers and introduced into a single optical fiber) is as high as 90%, the number of beams to be synthesized is limited since the angle of incidence of the beam bundle to the optical fiber is limited according to the numerical aperture (e.g., NA=0.2) of the optical fiber. That is, the power of the laser beams to be synthesized and introduced into the optical fiber is limited according to the numerical aperture of the optical fiber.
As a method of generating a plurality of laser beams having parallel optical axes, there has been known a technology which employs a plurality of semiconductor lasers arranged in one direction on one substrate. Since the active layers of the semiconductor lasers arranged in this way are arranged in flush with each other, the beam bundles radiated from semiconductor lasers arranged in this way having parallel slow axes in a plane. An assembly of a plurality of semiconductor lasers arranged in this way is sometimes referred to as “a laser bar”, and the direction of widths of the active layers of the semiconductor lasers perpendicular to the direction of the slow axes is the direction of fast axis of each beam bundle.
As a system of concentrating the largest possible number of laser beam bundles within the range of the angle of incidence determined by the numerical apertures of the optical fiber and synthesizing the laser beam bundles in the optical fiber, there has been known the following synthesizing system. FIG. 20A is a plan view showing a laser beam synthesizing apparatus as seen from above, FIG. 20B is a left side view of the laser beam synthesizing apparatus as seen in the optical axis of the laser beam bundle, and FIGS. 21A and 21B are views for illustrating the angle of convergence where FIG. 21A is a view showing a state where the whole beam bundle comprising a plurality of beam bundles is converged and FIG. 21B is a view showing an intensity distribution in the direction of slow axis in the whole beam bundle.
In the synthesizing system, the whole beam bundle comprising a plurality of beam bundles La, Lb, Lc, . . . radiated in the direction of arrow Z from a laser bar 1 comprising a plurality of semiconductor lasers 1A, 1B, 1C, . . . arranged in the direction of arrow Y perpendicular to the direction of arrow Z is passed through a cylindrical lens 2 (to be described later) and then passed through a converging optical system 6 to converge the whole beam bundle at an angle of convergence of α91 so that the width of the whole beam bundle as measured in the direction of the slow axis (the direction of double-headed arrow S, equal to the direction of arrow Y in this particular embodiment) is reduced. Then the whole beam bundle is converged on a converging position Pj which comes to be a point on the Y-Z plane positioned on a redirection system 7, that is, beam bundles making up the whole beam bundle are focused in different positions in the converging position pj which is a linear area extending in the direction of arrow X (the direction of the fast axis shown by double-headed arrow F) on the redirection system 7. The beam bundles La, Lb, Lc, . . . emanates from the redirection system 7 after directed by the redirection system 7 so that their optical axes become parallel to each other and in alignment with each other as seen in the direction of the fast axis (the direction of double-headed arrow F, equal to the direction of arrow X in this particular embodiment). Then the whole beam bundle is passed through a condenser optical system 3 and is converged at an angle of convergence of α92 (α92 is smaller than α91, here). Then the converged whole beam bundle is introduced into the core 5 of the optical fiber 4. There has been known a method in which the largest possible number of laser beam bundles are synthesized in one optical fiber in this manner. See, for instance, U.S. Pat. No. 6,462,883.
The angle of convergence is defined as follows.
On the basis of a particular position, a position (Yp, Zp) shown in FIG. 21A here, an intensity distribution (See FIG. 21B) in the direction of the slow axis (the direction of arrow Y) in the whole beam bundle about to converge is obtained. Then positions where the intensity is 0.1% of the maximum Pmax of the intensity in the intensity distribution are determined in the intensity distribution, and then outermost positions y1 and y2 which are the outermost in the direction of the slow axis (direction of the arrow Y) in the positions are determined. The space p between the positions y1 and y2 is calculated.
Further, the distance L in the direction of optical axis (the direction of arrow Z) between the position (Yp, Zp) and the converging position Pj of the whole beam bundle is determined.
The angle of convergence α is defined by the following formula.tan(α/2)=(φ/2) /L=φ/2L
The redirection system 7 can be formed by, for instance, superposing one on another in the direction of the fast axis a plurality of thin (small in thickness in the direction of arrow X or the direction of the fast axis) mirrors and one of the beam bundles La, Lb, Lc, . . . which are converged by the converging optical system 6 and are in different positions in the direction of arrow X is caused to impinge upon one of the mirrors, whereby the optical axes of the beam bundles are made to be aligned with each other as seen in the direction of the fast axis. “as seen in the direction of the fast axis” and “as seen in the direction of the slow axis” will be expressed “in the fast axis view” and “in the slow axis view”, hereinbelow.
More specifically, the laser beam bundles La, Lb, Lc . . . which are radiated in the same direction from the active layers 8A, 8B, 8C . . . formed in a plane in the laser bar 1 and whose slow axes are parallel in a plane are passed through the cylindrical lens 2 whose cylindrical axis (an axis determined in the direction in which the cylindrical lens extends) is inclined to the slow axes, and offset in different positions in the direction of the fast axis with the slow axes of the laser beam bundles La, Lb, Lc . . . kept parallel to each other, and the offset laser beam bundles are introduced into the redirection system 7 through the converging optical system 6. That is, the whole beam bundle comprising the laser beam bundles La, Lb, Lc . . . offset in different positions in the direction of the fast axis by the cylindrical lens 2 is converged by the converging optical system 6 so that the width in the direction of the slow axis is reduced, and at the same time, the laser beam bundles La, Lb, Lc . . . are converged in both the directions of the slow axis and the fast axis and are introduced into the redirection system 7 in different positions in the direction of the fast axis. The laser bar 1 is disposed on a block 9.
The beam bundles La, Lb, Lc, . . . are converged by the converging optical system 6 so that the optical axes of the beam bundles intersect each other at the converging position Pj in the fast axis view (that is, in the YZ plane) and the beam bundles are condensed in the converging position Pj.
Introduction of the beam bundles La, Lb, LC . . . into the optical fiber 4 through the redirection system 7 will be described, hereinbelow.
FIG. 22 is a plan view showing the optical axes and profiles of the beam bundles near the redirection system in an enlarged scale, FIG. 23 is a view seen in the direction of arrow G in FIGS. 20A and 20B or 22 showing the optical axes and profiles of the beam bundles near the redirection system in an enlarged scale, FIGS. 24A and 24B are views showing in an enlarged scale a state of the beam bundles emanating from the redirection system in a predetermined position to be described later and a state of the beam bundles introduced into the optical fiber as seen along the direction in which the beam bundles propagate, where FIG. 24A is a view showing a state of the beam bundles emanating from the redirection system and FIG. 24B is a view showing a state of the beam bundles introduced into the optical fiber, and FIGS. 25A and 25B are views for illustrating a state of the beam bundles when the redirection system is disposed out of the predetermined position as seen along the direction in which the beam bundles propagate where FIG. 25A is a view showing a state of the beam bundles emanating from the redirection system and FIG. 25B is a view showing a state of the beam bundles introduced into the optical fiber. In FIGS. 22 and 23, only the beam bundles La and Lc are shown and the other beam bundles Lb, Ld and Le are abbreviated.
As shown in FIGS. 22 and 23, the whole beam bundle is converged so that the converging position Pj is on the redirection system 7 and the beam waist of each beam bundle is on the redirection system 7. The redirection system 7 changes the direction of each beam bundle so that the optical axes of the beam bundles are aligned with each other in the fast axis view and radiates the beam bundles so that their optical axes are parallel to each other. Though the beam bundles radiated from the redirection system 7 subsequently diverge, the beam bundles are condensed again by the condenser optical system 3 and are introduced into the core 5 of the optical fiber A.
When the redirection system 7 is correctly disposed in a predetermined position as described above, the beam bundles radiated from the redirection system 7 are aligned with each other in the fast axis view and are linearly arranged as shown in FIG. 24A and the beam bundles introduced into the optical fiber 4 are also linearly arranged in the direction of the fast axis as shown in FIG. 24B.
On the other hand, when the redirection system 7 is disposed deviated from the predetermined position in the direction of arrow Z, the optical axes of the beam bundles radiated from the redirection system 7 are shifted in the fast axis view and are not linearly aligned with each other in the direction of the fast axis as shown in FIG. 25A, and the beam bundles introduced into the optical fiber 4 are not linearly aligned with each other in the direction of the fast axis but shifted from each other as shown in FIG. 25B. Accordingly, the beam bundles impinge upon a larger area of the end face of the optical fiber 4 as compared with when the optical axes of the beam bundles radiated from the redirection system 7 are linearly aligned with each other in the direction of the fast axis, and some of the beam bundles can impinge upon the end face of the optical fiber 4 outside the core 5. Accordingly, the coupling efficiency of the whole beam bundle to the optical fiber 4 deteriorates. In order to suppress the deterioration of the coupling efficiency, it is necessary to accurately dispose the redirection system 7 in a predetermined position in the direction of arrow Z.
As can be understood from the description above, it is necessary to accurately position and fix the redirection system in a very small space where a plurality of beam waists are concentrated and to produce the redirection system in a small size at a high accuracy to conform to the shape of each beam bundle. It is difficult to produce such a redirection system. Further, since the dimensions of the beam waist are proportional to the wavelength, as the dimensions of the beam waist become smaller with recent shortening the wavelengths of lasers, it becomes necessary that the redirection system is smaller in size and more accurately fixed, which makes it more difficult to produce the redirection system.
The laser beam synthesizing apparatus is for obtaining a laser beam bundle which is large in output (high in energy density) by synthesizing laser beams radiated from a plurality of semiconductor lasers since a small and large-output semiconductor laser is hard to realize, and accordingly, there has been a strong demand toward reduction of the size of the apparatus. For example, there has been a demand toward obtaining a laser beam synthesizing apparatus which is large in output for its size by reducing the size of the apparatus without reducing the output of the synthesized laser beam.
The system described above, where the whole beam bundle comprising laser beam bundles radiated from a plurality of semiconductor lasers and offset is converged and passed through a redirection system to align the propagating directions of the bundles with each other, and then the laser beam bundles are converged by a condenser optical system and introduced into the optical fiber to be synthesized into a synthesized bean bundle, is disadvantageous in that the optical path along which each of the beam bundles propagates from the semiconductor laser to the optical fiber is elongated, and at the same time, a number of optical parts including the lens for offsetting the beam bundles and the lens for converging the whole beam bundle must be disposed on the optical path, whereby the size of the apparatus is increased. Further, the system gives rise to a difficult problem that a smaller and more accurate redirection system must be provided as the wavelengths of the lasers are shortened.
Further, when the laser beam bundles radiated from a plurality of semiconductor lasers are offset by passing the laser beam bundles through a cylindrical lens whose axis is inclined to the slow axes of the beam bundles, the beam bundles passed through the periphery of the cylindrical lens become large in aberration. Since it is difficult to accurately converge the beam bundle having such a large aberration, e.g., to correctly introduce the beam bundle into the core (50 μm in diameter) of the optical fiber, the coupling efficiency deteriorates to, for instance, about 60%. In the case where a laser bar comprising an increased number of semiconductor lasers is used and an increased number of beam bundles are to be synthesized in an optical fiber in order to obtain a laser beam which is especially high in energy density, the length of the laser bar is increased and the offset must be increased, whereby the beam bundles passed through positions of the cylindrical lens largely deviated from the center of the cylindrical lens become large in aberration and it becomes difficult to obtain a high coupling efficiency.
Anyway, there is a problem that the range of incidence of laser beams which can be synthesized in a single optical fiber is limited by the numerical aperture NA of the optical fiber.